The present invention relates to a hologram recording device for recording interference fringes between signal light and reference light on a hologram recording medium, and particularly to a phase mask for suppressing the DC component of the signal light.
A holographic data storage system has recently been proposed which records and reproduces a large volume of data using hologram technology for volume-recording interference fringes between signal light spatially modulated by information and reference light on a hologram recording material.
In a conventional holographic data storage system, coherent laser light 50 emitted from a light source (not shown) is split into signal light 100 and reference light 200, as shown in FIG. 9. When the signal light 100 thus obtained is passed through a spatial modulator (SLM) 1, the signal light 100 is spatially modulated by information (data pattern) displayed in the SLM 1. The signal light 100 spatially modulated by the SLM 1 is condensed in the vicinity of a hologram recording material 3 by a lens 2, and is made to interfere with the reference light 200 separately applied to the hologram recording material 3, whereby a hologram is recorded.
At this time, the laser light 50 entering the SLM 1 is diffracted according to the pixel pattern of the SLM 1 and the data pattern being displayed in the SLM 1. As a result, the signal light 100 has a certain spread on the hologram recording material 3. According to a theory of wave optics, this spread has a relation to the Fourier transform pattern of an image displayed in the SLM 1, and the size of the spread is proportional to the size of pixels displayed in the SLM 1 and the focal length of the lens. FIG. 10 shows an example of simulation of the spread of the signal light 100 on the hologram recording material 3.
FIG. 10 indicates that light on the hologram recording material 3 has high intensities around an origin, and has low light intensities in other parts. Incidentally, the high intensity component around the origin is referred to as a DC component. Such a non-uniform distribution is known as problem of the DC component when the SLM 1 performing only intensity modulation is used. It is known that such non-uniformity causes various problems at a time of recording holograms, and some measures against the problems have been proposed (see for example Non-Patent Document 1, Holographic Data Storage; H. J. Coufal, D. Psaltis, and G. T. Sincerbox ED; Springer; p. 259 to 269 Beam Conditioning Techniques for Holographic Recording Systems).
One of such measures is a method using an optical part referred to as a random phase mask or a diffuser. This optical part for example has a same pitch as the pixel pitch of the SLM 1 and a random pattern with phase differences of zero and π. The optical part is disposed such that the random pattern of the optical part conforms to the pixels of the SLM 1. The optical part thereby diffuses light. The optical part having a random pattern with phase differences of zero and π is referred to as a phase mask. Thus diffusing light results in a uniform distribution of light on the hologram recording material 3, thus greatly improving recording characteristics.